A sparkgap intended to protect a circuit from the damaging effects of a high voltage surge by providing a shunting path for the surge prior to the circuit. The sparkgap determines the point within the circuit for transient or surge discharge and determines the specific potential of such a discharge. The location of desired discharge is determined by the circuitry involved, the source of such transients and the preferred discharge path. The differential voltage necessary for discharge is determined by the spacing between the two conductive points comprising the sparkgap. The required discharge potential is established by determining the transient or surge sources and the protection level desired, which, for example, may be determined by regulatory requirements.
Prior spark gap technologies include discrete sparkgaps (FIGS. 1A and 1B) or the more cost effective printed circuit board (PCB) sparkgaps (FIGS. 2A and 2B).
Discrete sparkgaps are generally constructed with leads terminating within a body of glass, ceramic or plastic at respective conductive surfaces with a fixed spacing therebetween. There are two categories of discrete spark gaps, one sealed with a controlled atmosphere as shown in FIG. 1A, and the other, unsealed at normal atmosphere as shown in FIG. 1B. Both of these types require a sizable amount of room for mounting (e.g., on a PCB) and both require careful physical placement and orientation during mounting due to spacing and other mechanical considerations. Discrete spark gaps, while tending to be more costly than printed circuit spark gaps, are somewhat more durable and normally exhibit a more controlled discharge potential.
It is well known to construct printed circuit boards (PCBs) comprising one or more layers of a rigid or flexible insulator (e.g., fiberglass or plastic) and one or more layers of a conductive material (e.g., copper or conductive ink) where various circuit components are electrically connected by conductive "traces."
As shown in FIGS. 2A and 2B, a PCB sparkgap utilizes conductive traces (e.g., copper) of the PCB which are positioned at a fixed distance from each other and a provide a controlled point for discharge of the unwanted transient or potential. The PCB sparkgap may have a slot punched in the board laminate between the nodes, depending upon the voltage potentials and energy involved. PCB sparkgaps are relatively easy to accurately and repetitively manufacture since there are few variables introduced through physical tolerancing and placement (in contrast to discrete sparkgaps). PCB sparkgaps provide predictable operation and are relatively inexpensive, costing perhaps little more than the actual cost of the board material.
A disadvantage of the printed circuit board sparkgap is evident upon repeated discharges, catastrophic events, or long term constant discharge. In these situations the printed circuit board surface and the copper foil quickly degrade with respect to the spacing between conductive nodes due to vaporization of foil. As the gap spacing increases the voltage potential required for an arc across the gap increases and the location of the arc may be less controlled. Additionally, the board laminate may sustain damage during catastrophic and long term discharge due to the intense heat which may result from such an event.
There are several techniques useful in improving the PCB sparkgap. For example, the problems associated with foil vaporization may be delayed by increasing the foil thickness or, in a PCB slot type of sparkgap, by increasing the length of the slot (while retaining the same width). Unfortunately, the area of the PCB required to implement a sparkgap increases with this technique and the vaporization will eventually degrade performance. Another technique is the use of multiple gaps spaced serially such that the energy of the spark is dissipated over two gaps (FIG. 2B). This technique also slows foil degradation at the expense of PCB area.
It is therefore seen to be desirable to provide a spark gap which eliminates or reduces the previously described limitations of prior art sparkgaps.